Twin insulator charge storage device operation and its fabrication method

ABSTRACT

The invention proposes am improved twin MONOS memory device and its fabrication. The ONO layer is self-aligned to the control gate horizontally. The vertical insulator between the control gate and the word gate does not include a nitride layer. This prevents the problem of electron trapping. The device can be fabricated to pull the electrons out through either the top or the bottom oxide layer of the ONO insulator. The device also incorporates a raised memory bit diffusion between the control gates to reduce bit resistance. The twin MONOS memory array can be embedded into a standard CMOS circuit by the process of the present invention.

This is a Divisional application of U.S. patent application Ser. No.11/059,080, filed on Feb. 16, 2005, which is a Divisional application ofU.S. patent application Ser. No. 10/685,873, filed on Oct. 15, 2003, nowissued as U.S. Pat. No. 6,900,098, which claims priority to U.S.Provisional Patent Applications Ser. No. 60/418,451 filed on Oct. 15,2002 and Ser. No. 60/436,129 filed on Dec. 3, 2002, which are hereinincorporated by reference in their entirety and assigned to a commonassignee.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The Invention relates to a high-density non-volatile memory device usingTwin-MONOS structure, and its fabrication method.

(2) Description of the Prior Art

An insulator charge storage device is a type of non-volatile memory inwhich charge is stored within the traps of an insulator material.Electrons may be injected into the insulator by either channel hotelectron (CHE) or tunneling. Electrons are usually eliminated via sometype of hole injection mechanism in a MONOS device contrasting to FNejection in a floating gate silicon device. In a MONOS device, nitrideis the storage element. When the bottom oxide is as thin as or less than23 Angstroms, holes are injected by a direct tunneling mechanism (S.Minami et. al., “A Novel MONOS Nonvolatile Memory Device Ensuring10-Year Data Retention after 10⁷ Erase/Write Cycles”, IEEE Transactionson Electron Device, VOL. 40, No. 11, November 1993, p.p. 2011-2017 andE. Suzuki, Y. Hayashi et. al., “Hole and Electron Current Transport inMetal-Oxide-Nitride-Oxide-Silicon Memory Structures”, IEEE Transactionson Electron Device, VOL. 36, No. 6, June 1989, p.p. 1145-1149) and theelectron negative charge is neutralized by the holes. When the bottomoxide is thicker than 30 Angstroms, high energy hot holes are generatedby band to band or avalanche breakdown; these holes are injected intothe storage area and recombine with the electrons to neutralize thecharge. (T. Y. Chan, Chenming Hu et. al. “A True Single-TransistorOxide-Nitride-Oxide EEPROM Device” IEEE Electron Device Letters, VOL.EDL-8, No. 3, March 1987, p.p. 93-95).

Hot hole injection is notorious for damaging oxide through injectionbecause its effective mass is three times larger than an electron's(Paulo Cappelletti et. al. “Flash Memories” Kluwer Academic Publishers1999, p.p. 217-223). This damaged oxide creates traps and reducesretention time. The retention time degradation increases asprogram/erase by hole cycle increases. (FIG. 1)

In his paper, K. T. Chang et. al. “A New SONOS Memory Using Source-SideInjection for Programming” IEEE Electron Device Letters, VOL. 19, No. 7,July 1998, p.p. 253-255, the author uses a split gate in an attempt toeliminate the trapped electrons by electric field applying positive biason the top polysilicon gate (FN erase) instead of hole injection and toavoid hole damage and to improve retention time. However this approachin sidewall split gate structure where the nitride layer is sandwichedbetween two polysilicon gates encounters the following problem.

FIG. 2 illustrates word gate 20 and control gate 22. Horizontal andvertical components of the ONO nitride 21 are designated as a storageelement and an insulator between the two polysilicon gates. The cornercomponent is off the control gate and is less controlled by the gate. Asmall number of electrons accumulate at the gap nitride 22 between thetwo-polysilicon gates 20 and 24 at every program erase cycle. In orderto eject the electrons trapped in the SiN, a positive bias is applied onthe control gate polysilicon 22 while the substrate silicon 10 isgrounded. Since the electric field at the gap is weaker compared to thearea immediately under the control gate, it is difficult to eject theelectrons trapped at the gap. FIG. 4 shows electrons 29 trapped in thenitride 21. Thus the erased state threshold shifts up as the number ofprogram/erase cycles increase, and the window between program and erasestates gets smaller. There is a reliability issue associated with thesplit gate approach. This is illustrated graphically in FIG. 3 wherelines 31 and 33 show threshold voltages of programmed and erased cells,respectively, as a function of number of cycles.

U.S. Pat. No. 6,498,377 to Lin et al describes a MONOS cell structurewhere nitride storage lies under sidewall spacers. U.S. Pat. No.6,356,482 to Derhacobian et al teaches applying a negative gate erasevoltage to improve erase after many program-erase cycles. U.S. Pat. No.6,040,995 to Reisinger et al discloses F-N tunneling erase of nitridethrough a thick oxide layer. U.S. Pat. No. 5,408,115 to Chang shows F-Ntunneling erasure through the top oxide wherein the bottom oxide isthick.

A Twin MONOS individual cell structure splitting the gate into one wordgate and two control gates on the word gate sidewalls was introduced inU.S. Pat. No. 6,255,166, by Seiki Ogura. Its fabrication method isdescribed in U.S. Pat. No. 6,531,350 also by Seiki Ogura et al. Thisinvention also refers to an array structure of 4 bit-1 contact describedin U.S. Pat. No. 6,469,935 by Y. Hayashi et al, where 4 memory storagecells share one contact. This invention still also refers to asimplified fabrication method described in U.S. Provisional PatentApplication Ser. No. 60/363,448, filed on Mar. 12, 2002, (docket numberHalo02-001), by K. Satoh et al.

The diffusion bit TWIN-MONOS array provided in U.S. Pat. No. 6,255,166contains two serious concerns. The ONO composite film is deposited afterdefining the memory word gate followed by the control gate process.Vertical ONO along the word gate sidewall and horizontal ONO overlyingthe substrate form the L-shaped ONO. There is a gap at the corner of theL-shape between the control gate and the nitride edge. This may make itmore difficult to pull the electrons stored in the corner. The electronsstored in the corner nitride are accumulated during program and erasecycles so that the operation window gets narrower as time goes on.

Another concern is the negative slope opening for defining the wordline. The word line mask 28 is patterned over the polysilicon line 26′overlying the polysilicon line 20′ as shown in FIG. 5A. The polysilicon26′ and 20′ not covered by the word line mask 28 should be etched out.The line 26′ has a positive slope but it becomes a negative opening foretching as shown in FIG. 5B. It is difficult to etch out the polysiliconunder the negative opening, as shown by poly residuals 23. It wouldeasily cause word line to word line short and word line to control gateshort.

SUMMARY OF THE INVENTION

The present invention provides a cell structure and array architectureof Twin-MONOS memory for high-density application and its deviceoperation to achieve this endurance of program-erase cycle to more than100,000 cycles and following retention time to longer than 10 years at85° C. The fabrication methods of the cell are also provided withsolutions for concerns in the prior arts.

The first embodiment of this invention is a device operation forL-shaped ONO to utilize hot hole erase in addition to F-N(Fowler-Nordheim) erase in order to improve the endurance. Holesgenerated by band to band can be injected into the gap region as long asthe control gate (CG) channel length is within the several hole meanfree path by applying a negative bias on the word gate (see U.S. patentapplication Ser. No. 09/810,122—Halo 00-004—Word gate negative holeinjection). Thus if hot hole damage is tolerable up to 1K cycles and hothole injection is required after 100 F-N erasures, then the endurancecycle providing the proper operating threshold voltage (Vt) windowextends to 100×1K=100 K cycles. However, this Hot Hole and CHEcombination approach is still limited by Hot Hole endurance comparing toCHE-F-N endurance.

The second embodiment of this invention is corner nitride free TwinMONOS. In the cell and array structure of the second embodiment, thewidth of the storage nitride is coincident with that of the control gateto prevent storing electrodes in the nitride under off-control gate suchas seen on a corner of the L-shape. A p-type species is doped in thecontrol gate polysilicon to eliminate electron source through the topoxide during F-N erase through the bottom oxide. An n-type species islightly doped in the control gate channel to prevent hot holeaccumulation during F-N erase. The bit diffusion is raised by fillingpolysilicon in between the control gates to reduce the bit resistance.The word gate opening is tapered with positive slope to allow word-linepatterning. The word gate is stepped down into the underlying channel toprevent short channel punch-through leakage for further advancedtechnology. The control gate runs along the bit diffusion and across theword line. The diffusion contact is placed at the end of every other bitdiffusion alternately in a memory array block, the control gate contactis placed on the extension of the bit contact and/or in-between the bitcontacts, and the word gate contact is placed at the end of the wordline alternately.

The fabrication method of the 2^(nd) embodiment consists of growing thebottom oxide on memory area, depositing nitride on the bottom oxide, anddirectly oxidizing the nitride with ISSG (Insitu Steam Generation)containing a higher hydrogen concentration than 2% to form the topoxide. The first deposited polysilicon on ONO film is used for thecontrol gate polysilicon. P-type species are implanted into the firstpolysilicon for F-N erase application. A cap nitride is deposited on thefirst polysilicon followed by etching with a word gate mask to the firstpolysilicon. An oxide spacer is formed on the sidewall of the capnitride as an etching mask to define the control gate and the ONOstorage element. The first polysilicon is etched with the oxide spacermask, followed by the control gate channel implant with angle, LDDimplant and dielectric spacer formation in between the control gate andthe diffusion. The second polysilicon is plugged in between and recessedto form a raised diffusion to reduce the bit line resistance followed byoxide fill and planarization. The first polysilicon under the capnitride is exposed by removing the nitride selectively followed byetching the first polysilicon and subsequently the ONO to define theother edge of the control gate and ONO storage as well as a positivelytapered opening. The substrate exposed after ONO etching may be etcheddown for further technology to prevent punch-through leakage due toshort channel. The word gate oxide is grown on the substrate and adielectric spacer is formed on the control gate sidewall for insulationto the word gate. The third polysilicon is deposited and patterned witha word-line mask, followed by the logic process.

The third embodiment is a modification of the second embodiment for bitapplication described in U.S. Pat. No. 6,469,935. The cell and arraystructure of the third embodiment contains modifications from the secondembodiment as follows. The memory cells are isolated by STI instead ofthe field implant in the second embodiment and the memory diffusion areais also isolated by STI. The memory diffusion is connected alternatelywith an upper or lower adjacent diffusion by local wiring to share acontact with 4 memory storages. The control gate runs along the wordgate and across the bit line connecting the bit contacts with metal.

The fabrication method of the second embodiment is modified for thethird embodiment as follows: the process steps through to the firstcontrol polysilicon etching are common with the second embodiment. Theword gate formation comes prior to the diffusion formation in theprocess sequence of the third embodiment. The word gate oxide is grownafter the first polysilicon etching. The word gate polysilicon isplugged and recessed in the word gate trench over the word gate oxidefollowed by cap oxide formation over the polysilicon such as raiseddiffusion formation in the second embodiment. The polysilicon exposed bystripping the cap nitride is etched down with the second control gateetching to form the memory control gate, where the polysilicon in thelogic area is also etched with a photoresist mask simultaneously. Thisis followed by memory channel implant, LDD implant and logic process,after filling and planarizing oxide over the diffusion area. A localwiring process is allowed to connect the adjacent diffusion. The contactformation and metal process follows.

The fourth embodiment is for higher density NAND application using MONOSmemory. The memory cell structure consists of subtracting the word gatefrom the third embodiment. It is simply replacing the polysiliconfloating gate of the conventional NAND cell by nitride. The unit cellalong the channel direction consists of a half of source/drain, acontrol gate with underlying ONO as an memory storage and other half ofsource/drain. The width of the control gate and underlying ONO isdefined by an overlying oxide sidewall mask. The unit cell dimensionalong the channel can be smaller than the conventional NAND. Thedirection across the channel is bounded by STI along the channel. Thearray structure follows NAND only replacing the floating gate bynitride. The bit lines run along the active area isolated by STI lines.The control gate lines are across the bit lines. A block consists ofevery a certain number of the control gate lines and bit lines. Thecontrol gates at both ends of the block are utilized as select gates todefine which block is operated. The diffusion area in between the blocksis shared as a bit diffusion connected to a bit line through contact andcommon ground, alternately. The control gate mask on a sidewall islooped. The looped mask is separated into two lines by cutting it atboth ends The control gate contact is also placed at the ends.

The device operation method of the fourth embodiment is designated byF-N program and F-N erase through the top oxide. This is different fromthe conventional NAND operation access through the tunnel oxide. Even inF-N ejection through the bottom oxide, a small number of hot holes aregenerated by band to band transition and accumulated at the channelsurface. The hot holes are easily injected through the bottom oxide andleave damage in the bottom oxide. This degrades the retention time afterprogram-erase cycles. On the other hand, F-N tunneling through the topoxide may not have a concern about such hot hole generation. The F-Ntunneling through the top oxide is considered to be more reliable thantunneling through the bottom oxide. Two device operation methods usingF-N tunneling through the top oxide are provided as program/erasedefined by electron ejection/injection and injection/ejection.

Multi level program method is also contained in the device operationmethods. Multi level program is to provide controllable memory cellthreshold voltage Vt at 4 or 8 levels by adjusting operation conditions.A memory cell with 4 level Vt is a two-bit cell so that the density ofthe two bit cell memory array becomes twice that of the single bit cellmemory array. The multi level Vt program is allowed by adjusting thecontrol gate voltage or bit line voltage.

The fabrication method of the fourth embodiment is designated byskipping the word gate polysilicon in the third embodiment. The processsteps through to sidewall oxide mask are the same as in the thirdembodiment. Stripping the cap nitride, only the looped oxide maskremains on the control gate polysilicon. The loop is cut at both ends ofa block into two lines. A photoresist mask for the control gate contactcover and logic gate is printed on the polysilicon. The polysilicon isvertically etched out with the sidewall oxide mask and the photoresistmask, followed by clearing ONO. The device impurity profile is definedby a LDD implant, spacer process, and source-drain implant. After oxidedeposition and planarization, the common source line and bit contact areformed by a damascene process.

The fifth embodiment in this invention is for NOR application using thesame cell structure and fabrication method as the fourth embodiment. Thecontrol gate having ONO storage runs across STI (active area). The bitline crosses the control gate. The diffusion area is formed on theactive area at both sides of the control gate. The diffusion area on oneside is connected to bit line through the bit contact. The diffusionareas on the other side are connected together as a common source line.An individual memory cell is addressed by selecting a bit line andcontrol gate.

The sixth embodiment is a single gate MONOS. The cell structure is ofsimply replacing the gate oxide of a conventional MOS FET by ONO. Thememory cell can have dual memory storage in nitride over the p-njunction. The memory cell structure is close to NROM proposed by B.Eitan et al, SSDM 1999. Proc, p522, but it is different in using a metalbit line instead of a diffusion bit line and STI instead of fieldimplant isolation. The memory cell can be fabricated with only one extramask compared to a conventional CMOS device. The memory cell is easilyembedded into a conventional CMOS platform. The array structure isderived from Twin MONOS metal bit application. The control gate andunderlying ONO run crossing the STI and active area. The bit line alsocrosses the control gate. Every other diffusion area bounded by the STIand the control gate is connected to the bit line through a contact.Program operation adapts electron injection into the nitride withchannel hot electron to store the electrons on each side independently.Erase operation is either with hot hole injection or F-N ejection. Therearises a concern about enduring the program-erase cycles. The differenceof mean free path between an electron and a hole causes mismatch oftheir injection profile along the channel. A few electrons remain at themiddle of the channel without being neutralized by hot holes. Theseelectrons are accumulated with the program-erase cycle so that thresholdvoltage is going up. It is a common concern with NROM. It is disclosedto inject hot holes from not only one side but also the other side ofthe channel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional representation of the prior art.

FIG. 1B is a graphical representation of threshold voltage as a functionof cycle time in a MONOS device of the prior art such as FIG. 1A.

FIG. 2 is a cross-sectional representation of a MONOS memory of theprior art such as described in the paper by K. T. Chang et al.

FIG. 3 is a graphical representation of threshold voltage as a functionof program erase cycles such as for the device of FIG. 2.

FIG. 4 is an enlarged view of the view in FIG. 2.

FIGS. 5A and 5B illustrate a problem of polysilicon residues in theprior art.

FIG. 6 is a graphical representation of the first preferred embodimentof the present invention in a MONOS device operation.

FIG. 7 is a cross-sectional representation of an enlarged portion of thesecond preferred embodiment cell structure of the present invention.

FIG. 8 is a cross-sectional representation of the second preferredembodiment array structure of the present invention.

FIGS. 9A through 20A and 22A show top views of the process flow proposedin the second preferred embodiment of the present invention.

FIGS. 9B through 20B, FIG. 21, and FIG. 22B show cross-sectional viewsalong A-A′ of FIG. 9A of the process flow proposed in the secondpreferred embodiment of this invention.

FIG. 9C to FIG. 20C show cross-sectional views along B-B′ of FIG. 9A ofthe process flow proposed in the second preferred embodiment of thisinvention.

FIG. 23A is a top view of an enlarged portion of the third preferredembodiment cell structure of the present invention.

FIG. 23B is a cross-sectional representation of an enlarged portion ofthe third preferred embodiment cell structure of the present invention.

FIG. 24 is a top view representation of the third preferred embodimentarray structure of the present invention.

FIG. 25A through FIG. 29A show top views of the process flow proposed inof the third preferred embodiment of this invention.

FIG. 25B to FIG. 29B show cross-sectional views along D-D′ in FIG. 25Aof the process flow proposed in the third preferred embodiment of thisinvention.

FIG. 25C to FIG. 29C show cross-sectional views along E-E′ in FIG. 25Aof the process flow proposed in the third preferred embodiment of thisinvention.

FIG. 29C 1 shows a cross-sectional view along F-F′ in FIG. 29A of theprocess flow proposed in the third preferred embodiment of thisinvention.

FIG. 30 is a top view of an enlarged portion of the fourth preferredembodiment cell structure of the present invention.

FIG. 31 is a cross-sectional representation of an enlarged portion ofthe fourth preferred embodiment cell structure of the present invention.

FIG. 32A 1, FIG. 32A 2, FIG. 32B 1 and FIG. 32B 2 are examples of deviceoperation of the fourth embodiment.

FIG. 33A through FIG. 38A show top views of the process flow proposed inof the fourth preferred embodiment of this invention.

FIG. 33B to FIG. 38B show cross-sectional views along A-A′ in FIG. 33Aof the process flow proposed in the fourth preferred embodiment of thisinvention.

FIG. 33C to FIG. 38C show cross-sectional views along B-B′ in FIG. 33Aof the process flow proposed in the fourth preferred embodiment of thisinvention.

FIG. 39A through FIG. 40A show top views of the process flow proposed inof the fifth preferred embodiment of this invention.

FIG. 39B to FIG. 40B show cross-sectional views along A-A′ in FIG. 32Aof the process flow proposed in the fifth preferred embodiment of thisinvention.

FIG. 39C to FIG. 40C show cross-sectional views along B-B′ in FIG. 32Aof the process flow proposed in the fifth preferred embodiment of thisinvention.

FIG. 41 is a top view of an enlarged portion of the sixth preferredembodiment cell structure of the present invention.

FIG. 42 is a cross-sectional representation of an enlarged portion ofthe sixth preferred embodiment cell structure of the present invention.

FIG. 43 is a cross-sectional representation of the cell operation of thesixth preferred embodiment of the present invention.

FIG. 44 is a graphical representation of the cell operation of the sixthpreferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

It is implemented for device operation of the first embodiment to insertone hot hole erasure after every n (100 or 1000) cycles of CHE programand F-N erase as shown in FIG. 6. Hot holes generated by band to bandtransition can be injected into the nitride portion at the L-shapedcorner and neutralize the accumulated electrons there as long as thecontrol gate (CG) channel length is within the several hole mean freepath by applying a negative bias on the word gate. (See U.S. patentapplication Ser. No. 09/810,122—Halo 00-004—Word gate negative holeinjection). Therefore if hot hole damage is tolerable up to 1K cyclesand hot hole injection is required after 100 F-N erasures, then theendurance cycle providing the proper operating threshold voltage (Vt)window is extended to 100×1K=100K cycles. However, this Hot Hole and CHEcombination approach is still limited by Hot Hole endurance comparing toCHE-F-N endurance.

The device cross-sectional structure of the second embodiment along the1^(st) direction is shown in FIG. 7 as well as the top view of arrayarchitecture shown in FIG. 8. The bit diffusion is in between a pair ofthe control gates partitioned by memory spacer 115. The bit diffusionconsists of memory diffusion 103 and overlying raised diffusion 141. Thecontrol gate consists of the control gate polysilicon 140, underlyingONO (bottom oxide 111, storage nitride 112 and top oxide 113) andunderlying control gate channel 101. The pair of the control gates areconnected to each other at the end of diffusion bit. The control gateand the bit diffusion run along the 2^(nd) direction crossing the 1^(st)direction. Their contacts are placed at the end of the bit diffusion.The word gate consisting of word gate polysilicon 142, underlying wordgate oxide 118 and underlying substrate 100 is on the other side of thecontrol gate partitioned by CG-WG isolation dielectrics 118. The wordgates are connected by a word line 143 along the first direction. Theword line contact is placed at the end of the word line alternately.

The width of storage nitride 112 is coincident with that of the controlgate 140. L-shaped nitride 29 in previous art FIG. 4 has off controlgate at the corner. The electric field at the corner is weak compared tothe area immediately under the control gate so that it is difficult toeject the electrons trapped in the off control gate nitride at thecorner with F-N current and electrons remaining at the corner areaccumulated with the cycle of program and erase cycle. The thresholdvoltage of the control gate gets higher with the accumulation.Eliminating the off control gate nitride is promising to improve theendurance for the program-erase cycle.

The control gate polysilicon 140 is doped with p-type species instead ofn-type to help F-N ejection through the bottom oxide, where negativevoltage is applied on the control gate and positive or 0 voltage on thesubstrate so that electron ejection through the bottom and electroninjection through the top oxide can occur simultaneously with someprobability. It makes F-N ejection hard. Since p-doped polysiliconcannot be an electron donor, electrons are not supplied fromp-polysilicon of the control gate. and electron injection through thetop oxide is cut off.

An n-type species may be lightly counter-doped under the control gate inaddition to p-type dopant. Hot holes are generated even under F-N ejectconditions. The holes are accumulated along the channel under thecontrol gate and some are injected through the bottom oxide during F-Nerase. This damages the bottom oxide and degrades the data retentionafter cycling. An n-type dopant prevents the hot hole accumulation andinjection.

The memory diffusion 103 is shared by adjacent control gate pair 140 andadjacent word gates 142 on both sides as a common bit. The polysilicon141 plugged in between the control gates immediately over the diffusiondefines a raised diffusion. The resistance of the diffusion 103 is toohigh to work as a common bit line. The diffusion is covered by oxide 116before the logic (peripheral) gate process. It is difficult to share thesalicide process with the logic gate. The raised diffusion 141 byfilling polysilicon immediate over the diffusion is devised to lower thebit line resistance.

The word gate 142 opening is tapered with a positive slope. The wordline is defined by etching polysilicon 26′ in FIG. 5A along the firstdirection as shown in 5B. The negative such as in FIG. 5B would be amask for etching. It is so hard to etch out the polysilicon inner wordgate that poly residuals 23 remains along the word gate corner as shownin FIG. 5B. It may cause word-word short or word-CG short. The negativeslope comes from defining the word gate as remained pattern. It isdefined by trench pattern with positive slope opening in thisembodiment, followed by filling polysilicon and patterning the wordline.

The word gate 142 may also be stepped down into the underlying channel.Punch through current due to short channel has recently been garneringserious attention. The channel length of Twin MONOS is figured out by 2×control gate width+word gate width. It has a benefit in short channelbecause of the 2× control gate width. However, serious problems willoccur even in Twin MONOS in the 0.09 um and beyond era. The shortchannel punch through can be controlled by stepping down the word gateinto the channel by a few nm.

The array architecture of the second embodiment is for diffusion bitarray organization (U.S. Pat. No. 6,255,166B1). The control gate 140 andbit line 141 run along the 2^(nd) direction crossing the word line 143along the 1^(st) direction. in FIG. 7. The diffusion contact 161 isplaced at every other end of the bit diffusion 141 alternately in amemory array block, as illustrated in FIG. 8. The control gate contact160 is placed on an extension of the bit contact (160-1) and/orin-between the bit contacts (160-2). The word gate contact 162 is placedat the end of every other word line alternately.

Referring now to FIGS. 9A, 9B, and 9C, shallow trench isolation (STI)110 is formed within the substrate 100. STI is placed under the plannedmemory control gate contact and memory word line contact and as logicdevice isolation. No STI is under the memory cell array. Individualcells are isolated by field implantation after they are defined. An ONOcomposed film runs under the control gate.

A triple well structure is required in addition to conventional N-Welland P-Well structures to supply negative voltage. After patterning theresist with Deep N-Well mask, Phosphorus is implanted with the energy ofbetween about 1.5 MeV and 3 MeV to twice the depth of the inside P-Well.P-well in the triple well is formed commonly with the standard P-well.The well structure is not shown in the figures.

The 1^(st) gate oxidation and 2^(nd) gate oxidation processes aresubsequently allowed with base CMOS parameters. Logic thick gate oxideis designated as a combination of 1^(st) and 2^(nd) oxidation. The logicthin gate oxide, memory word gate, and ONO bottom oxide is grown at the2^(nd) oxidation.

An ONO stack film as a memory storage element is composed of the baseoxide 111, the storage nitride 112 and the top oxide 113 as shown inFIG. 9B. The base oxide is thermally grown to the thickness of betweenabout 2 to 6 nm together with memory work gate and logic thin gate,followed by standing the wafer in an NH₃ ambient at >850° C. to allownitridation on the surface. Thickness of the logic gate and memory wordgate is adjusted in a later process. The nitridation process helps notonly to reduce incubation time and deposit nitride uniformly but also toadjust the surface state of the bottom oxide—nitride boundary. As anexample, the nitridation time can be prolonged to increase the electrontrap sites on the surface. Nitridation time can be adjusted depending onwhat operation is required. The nitride thickness to be deposited by aconventional chemical vapor deposition (CVD) tool depends on the topoxide formation, either deposition or oxidation of the nitride. Thefinal thickness sandwiched between bottom and top oxides is controlledto be between about 2 and 6 nm. In the case of depositing the top oxide,the nitride thickness is required to be between about 2 and 6 nm. Thetop oxide is deposited to a thickness of between about 3 to 7 nm by aconventional CVD such as high temperature oxide (HTO). It also has to befollowed by a wet oxidation process to stabilize the boundary surface ofthe top oxide and the nitride. In the other case of oxidizing thenitride, the nitride thickness is figured out as between about 4 and 9microns to compensate for the thickness loss of the nitride from theoxidation. The thickness of about 3 to 5 nm of nitride turns into 4.5 to7.5 nm of oxide during the oxidation. An in-situ steam generation (ISSG)tool is preferred as an oxidation tool to a conventional wet oxidationwith a furnace to minimize the effect on other than ONO. When ISSG isadopted, nitride oxidation is shared with the logic gate oxidation. Thecombination of the three thicknesses is carefully chosen considering theoperation mode. For example, when electrons stored in the nitride areerased through the bottom oxide, the bottom oxide should be thinner thanthe top oxide. For erasing through the top oxide, it is vice versa.Hydrogen atoms contained in the process gas improve the data retention.Hydrogen concentration in ISSG is controlled by higher than 2% volume.

There are some options to reduce the F-N erase voltage. NH₃ anneal orN₂O anneal at 900° C. over ONO stack can be an option. These anneals maylower work function at the nitride—top oxide boundary. Another option issilicon rich nitride by increasing SiH₄ or SiH₂Cl₂ flow compared to NH₃flow at nitride deposition. The option works to reduce the deep trapslocating both boundaries with top and bottom oxide as well as increasethe shallow traps generated in bulk nitride. Another option is NO annealat 900° C. after nitride deposition or 700° C. H₂ anneal after thedevice is defined. These anneals reduce dangling bonds in the nitride.These options can be added in the preparation of the ONO film duringdevice fabrication to reduce residue electrons and to lower erase statevoltage.

The top oxide and nitride of ONO film in the logic area is removed withmasking the memory area, followed by logic gate oxidation to adjustlogic gate thickness.

Referring now to FIGS. 10A, 10B, and 10C, polysilicon 140 is depositedto the thickness of between about 100 to 200 nm followed by n-channelpolysilicon implantation into memory and N-MOS areas. P-type species maybe implanted into the polysilicon in the memory area to allow F-N eraseoperation. The cap nitride 119 is deposited over the polysilicon 140surface with resist mask in the memory area. The logic area is coveredwith a resist mask during etching. The resist mask is stripped. Oxide114 is deposited to a thickness of between about 20 and 80 nm to definethe width of the control gate, then it is vertically etched to thepolysilicon to define an oxide control gate mask. The oxide 114 over thelogic cap nitride 119 is etched away during the oxide etching.

Referring now to FIGS. 11A, 11B, and 11C, polysilicon 140 is etched tothe ONO surface using the oxide mask 114. CG contact area 170 and WGcontact pad area 172 are masked with photoresist. Removing the ONOgently, Boron or BF₂ is implanted with a tilted angle into the substrateunder the control gate to form a control gate channel 101. As an option,n-type implantation such as Arsenic may also be added to the channel 101with a lower dose than p-type. The n-specie works to prevent holt holeaccumulation along the CG channel during F-N erase. N-type specie isimplanted without a tilt angle to create memory LDD region 102 and PNjunction.

Referring to FIGS. 12A, 12B, and 12C, an oxide film 115 is deposited toa thickness of between about 30 and 60 nm and vertically etched to forman insulation of the control gate to the subsequent raised diffusion 141and source/drain offset. The control gate contact area 170, diffusioncontact area 171 and the word gate contact area 172 are masked withphotoresist 178 to retain the oxide 115. There is retained oxide 179 onthe cap nitride as shown. It would be etched out at oxide cap CMPprocess, otherwise it becomes a mask at the word gate opening process.FIGS. 12B1, 12C1, 12B2, 12C2, 12A3, 12B3 and 12C3 explain an option notto leave the oxide 115 on the cap nitride 119 after the spacer etch. Thephotoresist 178 is patterned over the oxide 115 to cover the controlgate contact area 170, diffusion contact area 171 and word gate contactarea 172 as shown in FIG. 12B 1 and FIG. 12C 1, as the pattern edge isplaced on the cap nitride 119. The resist is ashed back until thepattern edge comes to off cap nitride as shown in FIG. 12B 2 and FIG.12C 2. Pad areas are covered with the resist but no resist remains onthe cap nitride. Then, the exposed oxide 115 is vertically etched toleave the spacer oxide and pad protects. Source/drain implant 103 is anoption in case the dopant provided from the raised polysilicon is notenough. It can be adjusted appropriately.

Referring now to FIGS. 13A, 13B, and 13C, polysilicon 141 with athickness of between about 100 nm and 200 nm is deposited to plug thetrench 180 between the gates. The contact areas 170, 171, 172 arecomparably large to plug the polysilicon 141 so they are covered withoxide 115 to prevent etch in. The polysilicon 141 is vertically etchedto recess by 50 to 100 nm from the top of the cap nitride 119 as shownin FIGS. 14A, 14B, and 14C. An n-type specie is doped by ion implantafter the polysilicon recess. About 50 to 150 nm of oxide 116 isdeposited and etched down with vertical ion etching or CMP to planarizethe top surface, as shown in FIGS. 15A, 15B, and 15C.

Referring now to FIGS. 16A, 16B, and 16C, the remaining cap nitride 119is removed with a wet etch or CDE (Chemical Down Flow Etching). Thecontrol gate 140, the raised diffusion and logic gate 141 are coveredwith oxide 114 and 116. The word gate trench 142 in between the controlgates and logic area are now exposed for etching.

Referring to FIGS. 17A, 17B, and 17C, the vertical polysilicon etchingis allowed without photoresist mask. The polysilicon 140 other than thatcovered by the oxide 114 over the control gate and 116 over the raiseddiffusions is etched out. The remaining ONO and the exposed logic gateoxide are gently removed. Therefore ONO memory storage is definedself-aligned to the overlying control gate 140 as shown in FIG. 17B.About 20 to 40 nm of oxide 118 is deposited and vertically etched toassure isolation between the control gate 140 and the word gate 142. Astep word gate is sometimes preferred to reduce punch through leakagearising from a short channel. A step word gate lower than the controlgate is formed by etching the substrate lightly in this process step asan option as shown in FIG. 17B 1.

The word gate oxide 118 is grown over the channel area with a thicknessof about 3 to 15 nm, as shown in FIGS. 18A, 18B, and 18C. The logic gateis wrapped with oxide. The control gate slope is positive and the wordgate trench opening is also positive. It is convenient to fill and/oretch out the polysilicon in the trench.

Referring to FIGS. 19A, 19B, and 19C, another polysilicon layer 143 isdeposited to fill the word gate trench 142 to the thickness about 80 to150 nm. The word line 143 is patterned horizontally, crossing the wordgate trench 142 by a conventional lithography. The polysilicon under thespace 181 between the word lines in FIG. 18A is etched out as shown inFIGS. 18B and 18C.

Then the word line 143 including word line contact pad 172 is completed.During the word line process, the logic area is masked with photoresist.A field implant of B or BF₂ with a dose about 1E14 is allowed to isolateindividual memory cells. It is followed by the conventional logicprocess. The word line space 180 is plugged by a logic spacer. The wordline 143 may see logic salicidation.

There are three more variations in the fabrication method of the secondembodiment. The 1^(st) variation is to define the CG hard mask 114 aftercap nitride strip as shown in FIG. 21 instead of the step after capnitride mask described in FIG. 10B. CG mask 114 does not have to beoxide.

The 2^(nd) variation is to adapt thick oxide protection 117 on thecontact covers for the control gate and the word line after depositingthe 1^(st) polysilicon 140 as shown in FIGS. 22A, 22B, and 22C. Thephotoresist process in FIG. 11B is not necessary with the thick oxide.

Another option is to define the logic gate with CG polysilicon 140instead of the word line polysilicon 143. The logic area is covered withphotoresist at word gate trench etching.

The device cross-sectional structure of the third embodiment along the1^(st) direction is shown in FIG. 23B as well as the top view in FIG.23A. The array architecture is shown in FIG. 24. The cell structure is amodified structure of the second embodiment for a metal bit application.A unit cell is bounded by STI 210 in the second direction and by bothsides of diffusions shared by adjacent unit cells in the firstdirection. The diffusion area 202 also isolated by the STI is connectedto an adjacent diffusion alternately by local wiring 241.

The local wire is shared by 4 memory elements under the control gate.The word gate 242 and the control gate 240 run together along the 2^(nd)direction. The word gate becomes the word line. The local wiring 241 isconnected to 1^(st) metal 252 through bit line contact 251 as shown inFIG. 24. The 1^(st) metal is designated as a bit line so that thearchitecture was named as metal bit architecture. The word gate contact261 and its cover 271 are placed on the end of word line 242, likewiseplacing the bit line contact 161 and its cover 171 in the secondembodiment. The placement of the control gate is identical with thesecond embodiment.

The fabrication method of the third embodiment is mostly derived fromthe second embodiment. It is mainly different from the second embodimentin process sequence to define the word gate prior to the diffusion.Referring to FIG. 25A, the memory active area is defined by straight STI210 using a conventional process to isolate the memory cell along the2^(nd) direction. STI is also created in memory-logic boundary andcontact area of the word line and the control gate as shown in FIGS.25A, B and C. The process steps through ONO formation, gate oxideformation, deposition of the control gate polysilicon 240, cap nitride219 deposition, cap nitride mask, and oxide CG mask formation to thecontrol gate polysilicon are copied from the second embodiment as shownin FIG. 9 to FIG. 11.

Referring to FIGS. 26A, B and C, CG-word isolation 216 is formed on thecontrol gate polysilicon and the word gate oxide 217 is grown after the1^(st) CG polysilicon etching likewise as described in the secondembodiment.

Referring to FIGS. 27A, B and C, the word gate polysilicon 242 isdeposited to be plugged in the word gate trench. The polysilicon isrecessed down as in the raised diffusion process in the secondembodiment. The oxide 218 is deposited and planarized by CMP or etchingback.

Referring to FIGS. 28A, B, and C, cap nitride 219 is stripped by wetetching or dry etching and polysilicon 240 under the cap nitride isexposed. The photoresist is patterned on the logic area polysilicon. Thepolysilicon 240 is vertically etched to ONO. ONO is subsequently gentlyetched out. The CG channel implant and LDD implant are allowed with thesame conditions as in the second embodiment, then followed by memoryspacer 215 and memory source/drain implant 203.

Referring to FIGS. 29A, B, C and C1, FIG. 29C is a cross section alongword gate polysilicon 240 (cross-section G-G′) and FIG. 29 C1 is alongthe word gate space (cross-section F-F′). After oxide planarization,adjacent every pair of diffusions are connected together by local wiring250 as shown in FIG. 29C 1. The bit contact 251 is paced on the centerof the local wiring 250. The bit contacts are connected by the 1^(st)metal 252. The control gate contact 260 and the word contact 261 areplaced at the end of the word line as shown in FIGS. 29A and C. Thethird embodiment is therefore completed.

A top view and a cross sectional structure of the fourth embodiment areshown in FIG. 30A and FIG. 30B. It is simply replacing the floating gatein the conventional NAND by nitride 312 besides reducing the cell size.The memory cell structure is of subtracting the word gate 242 in FIG.23B of the third embodiment and forming a diffusion area instead. Theunit cell is shown in FIG. 30 as a rectangular dotted line. It consistsof a half of a diffusion area combining memory LDD 302 and source/drain303, a control gate with underlying ONO 311/312/313 as a memory storageand the other half of the diffusion area along the channel direction.Crossing direction is bounded by conventional STI 310. The oxidesidewall mask 314 defines the width of the control gate and underlyingONO to between about 30 nm and 60 nm. It is much smaller than thecontrol gate width of the conventional NAND. The direction across thechannel is bounded by STI along the channel. The array structure followsNAND with the only difference being replacing the floating gate bynitride. The bit lines run along the active area isolated by STI lines.The control gate lines are across the bit lines. An operation block isdefined as a (n bit lines×m control gate lines) matrix. The control gatelines 370B, 370S at both ends of the block are assigned as gates toselect a block to be operated. Two adjacent blocks share a diffusionarea in between either as a common source line adjacent to the selectgate 370S or a data bit adjacent to the other select gate 370Bconnecting to a bit line 351 through a contact 350. The control gatemask 314 is a sidewall image. The sidewall image loops around. Thelooping mask is separated into two lines by cutting it at both ends. Theadjacent control gate lines of adjacent loops are cut at the controledge cut 381 alternately each end. The control gate contact cover 380 isplaced on the out side of edge cut 381.

Two device operation methods of the fourth embodiment are provided,utilizing F-N tunneling through the top oxide. The first method is forprogram to eject electrons from the nitride and for erase to injectelectrons into the nitride with the voltage condition as shown in FIG.32A 1 and FIG. 32A 2. The common source line is always grounded. Theprogram operation is allowed by applying a high voltage on a gate and alow voltage on a channel of a selected memory cell to eject electronsstored in the nitride through the top oxide. Memory cell Vt (thresholdvoltage) shift is controlled by a difference of provided voltages on thegate and the channel. The difference is adjusted by the channel voltageprovided from the bit line or the gate voltage and enables the multilevel memory cell with controllable Vt at 4 or 8 levels. A memory cellis selected by a bit line 351 in FIG. 30, a control gate line 371 and apair of selected gates 370B at the bit side and 370S at the source side.The selected bit line voltage is adjusted between 0 to 3V to control theVt at multi level. 6V is provided on the bit side select gate andunselected control gates to pass the bit line voltage under the gates.The selected control gate is controlled at 13V. The difference between13V on the gate and the bit line voltage passing through to the channelshifts the cell Vt to the required voltage level. The difference betweenthe unselected cell gate and the channel does not shift the Vt. 4V isprovided to unselected bit lines to lower the voltage difference betweenthe selected gate and the unselected bit line to prevent programming.The erase operation is allowed on the whole block. All the control gatesare connected to −13V and the pair of select gates and substrates areconnected to 0V to inject F-N electrons into the nitride through the topoxide.

The second operation method of the fourth embodiment is for program toinject electrons into the nitride and for erase to eject electrons fromthe nitride with the voltage condition as shown in FIG. 32B 1 and FIG.32B 2. The program operation is allowed by applying a low voltage on agate and a high voltage on a channel of a selected memory cell to injectelectrons into the nitride through the top oxide. The memory cell isselected by a bit line, a control gate line and a pair of selected gatesat the bit side and at the source side. The selected bit line voltage isadjusted between 0 to 3V to control the Vt shift. 6V is provided on thebit side select gate and all the control gates to pass the bit linevoltage under the gates, followed by lowering the selected control gateto −13V to inject electrons. The difference between 10V on the gate andthe channel voltage passed from to the bit line shifts the memory cellVt to the required voltage level. The difference between the unselectedcell gate and the channel is too small to shift the Vt. 4V is providedto the unselected bit lines to lower the voltage difference between theselected gate and the unselected bit line to prevent programming. Theerase operation is allowed on the whole block. All the control gates areraised to 13V, the bit select gate is 0V to shut down the bit voltage,and the source select gate is opened with 6V to pass 0V to the channels.

The fabrication method of the fourth embodiment is featured to skip theword gate process in the third embodiment. Referring to FIGS. 33A, B,and C, the process steps through to sidewall oxide mask are common withthe third embodiment. ONO stack film, bottom oxide 311/nitride 312/topoxide 313, the control gate polysilicon 340, and cap nitride 316 aredeposited with STI 310 running horizontally in the memory array. The capnitride is patterned as crossing the STI with conventional lithographyand vertical etching to polysilicon 340. CG oxide 314 is deposited andvertically etched to form the control gate mask. Though it was followedby vertical polysilicon etching in the third embodiment, the polysiliconetching is skipped at this step in the fourth embodiment.

Referring to FIGS. 34A, B, and C, the cap nitride 316 is stripped. Thereremains only the looped oxide mask on the control gate polysilicon.Referring to FIGS. 35A, B, and C, the looped mask is selectively cut atboth ends of a block into two lines using photoresist mask opening thearea 381. Adjacent control lines of adjacent loops are cut alternately.

Referring to FIGS. 36A, B, and C, a photoresist mask for the controlgate contact cover 380 and logic gate is printed on the polysilicon. Thepolysilicon 340 is vertically etched out with the sidewall oxide mask314 and the photoresist mask to top oxide 313 surface. Nitride 312 andbottom oxide 311 is gently stripped out. LDD is implanted with the sameconditions as in the second embodiment. The angled channel implant inthe second and third embodiments may not be necessary.

Referring to FIGS. 37A, B, and C, the spacer dielectric film 315 such asnitride is deposited and vertically etched to define source/drain offset302. The Source/Drain is implanted. Oxide 317 is deposited to plug thesource/drain canyon and planarized with CMP.

Referring to FIGS. 38A, B, and C, the common source lines 330 to connectunderlying diffusion area and bit contact 350 are formed by a tungstendamascene process, individually or simultaneously. Another oxide isdeposited. The bit line contact 350 and the control gate contact 360 areformed by a conventional contact process. The first metal 351 connectsthe bit contact along the bit line. It is followed by the conventionalinterconnection process and completed.

The fifth embodiment in this invention is to modify the fourthembodiment to NOR application to access the individual bit randomly. Itmay only modify the arrangement of the common source lines 330 formed bylocal wiring and the bit contacts 350 as shown in FIGS. 39A,B,C andFIGS. 40A,B, C. The diffusion areas on one side of the control gate 340are connected together with a local contact to make it a common sourceline. The diffusion areas on the other side are connected to the bitline through the bit contact. An individual memory cell is addressed byselecting a bit line and control gate.

After FIGS. 37A, B, C, trenches 330 are opened in every other oxide 317between adjacent control gates with conventional lithography andvertical etching to diffusion area 303. Titanium nitride and tungstenare filled in the trench. The excess TiN and Tungsten are removed by CMPto form the common source lines 330. Another oxide 319 is deposited onthe planarized surface. The bit contacts 350 are opened over the bitdiffusion adjacent to the source lines together with the control gatecontact 360 and logic contacts. The bit contacts are connected with1^(st) metal bit line 351.

The sixth embodiment is a single gate MONOS as shown in FIG. 41. Thememory cell structure is close to NROM. The memory cells have dualmemory storage over the p-n junction 401 at each edge of the channel.The cell isolation is STI, different from field implant isolationadapted in NROM. It doesn't have the buried diffusion in NROM. It isderived by only replacing the gate oxide of a conventional MOS FET byONO 411/412/413.

The array structure of the sixth embodiment is shown in FIG. 41 and FIG.42. The control gate 440 and underlying ONO run crossing STI 410 andactive area 400. The bit line 452 also crosses the control gate.

The diffusion area is bounded by the STI 410 and the control gate. Everyother diffusion is connected to the bit line through contact 450. The2^(nd) structure is shown in FIG. 41 and FIG. 42. Two metal bit linesare in one STI/Active pitch. Though the active area is wider than priorembodiments and affected by area penalty, the fabrication is simplerthan others. It needs only one extra mask compared to base CMOStechnology.

Program operation adapts electron injection into the nitride withchannel hot electrons to store the electrons in each side independently.Erase operation is either with hot hole injection or F-N ejection. Therearises a concern about enduring the program-erase cycles with thisstructure. Remaining electrons in the middle of the channel that are outneutralized by hot holes are accumulated during the cycle. The controlgate Vt is going up. It is one of the solutions to inject hot holes fromnot only one side but also from the other side of the channel as shownin FIGS. 43 and 44.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

1. A method of device operation of a twin MONOS memory comprising:polysilicon word gates having polysilicon control gates on sidewalls ofsaid word gates having an L-shaped oxide-nitride-oxide (ONO) layerbetween said polysilicon word gates and said polysilicon control gatesand underlying said control gates wherein said nitride portion of saidONO layer underlying said control gates provides memory storage; whereinsaid method of device operation comprises: performing one hot holeerasure after every n cycles of channel hot electron (CHE) programmingand Fowler-Nordheim (F-N) erasure wherein electrons injected by said CHEprogramming are not perfectly erased by said F-N erasure and remainingsaid electrons are accumulated with said cycles and wherein said hotholes are generated by band to band transition by applying a negativebias on said word gate and are injected into said nitride portion ofsaid ONO layer not only directly under one of said control gates butalso at a corner of said L-shaped ONO layer wherein said hot holesneutralize accumulated said electrons within said nitride portion ofsaid ONO layer at said corner of said L-shaped ONO layer.
 2. Theoperation method according to claim 1 wherein said n is 100 to 1000cycles.
 3. The operation method according to claim 1 wherein the controlgate channel length is within the several hole mean free path.